Duobinary coding is advantageously used in transmission systems in order to reduce the transmission bandwidth or to increase the transmission capacity. In the case of duobinary coding, a special case of partial response binary coding, a binary data signal is converted to a three-stage signal using a specific coding rule. The bandwidth requirement for this ternary transmission signal is, in consequence, halved in comparison to that of a binary data signal. The code, which was originally developed for transmission of electrical signals, is also used for the transmission of optical signals, owing to these characteristics.
The use of this code is also explained for optical signal transmission in the journal “Journal of Lightwave Technology”, Vol. 16, No. 5, May 1998, pages 788 to 797, “Characteristics of Optical Duobinary Signals in Terabit/s Capacity, High-Spectral Efficiency WDM Systems”, Takashi Ono et al. The duobinary signals are converted to an amplitude-modulated and phase-modulated optical transmission signal. Two duobinary (ternary) values are transmitted via one high signal level and two different phase angles, with the carrier signal being at zero level, or at only a low level, for a third value. In principle, it is also possible to transmit other states but these are generally less advantageous for optical systems. The duobinary coding can be carried out via a precoder and at least one electrical filter, after which conversion to an optical signal is carried out via an optical modulator; for example, a Mach-Zehnder modulator.
An arrangement for duobinary coding using a precoder and two electrical low-pass filters, via which an optical modulator is driven, is described in Electronics Letters Jan. 18, 2001, Vol. 37, No. 2, pages 109 to 110 “320 Gbit/s WDM repeaterless transmission using fully encoded 40 Gbit/s optical duobinary channels with dispersion tolerance of 380 ps/nm” by K. Yonenaga et al. However, it is difficult to drive the modulator satisfactorily in both directions via two identical signals at a high transmission rate of approximately 40 Gbit/s. Furthermore, electrical coding of the signal results in the optical signal downstream from the nonlinear Mach-Zehnder modulator being spectrally broadened, thus reducing the bandwidth efficiency. The electrical low-pass filters are required for each modulator.
An object of the present invention, therefore, is to specify a method and a system for duobinary coding with improved signal quality. A method and a system for wavelength division multiplex systems (WDM systems), which can be implemented with little effort, is desired as well.